Between the years of 1923 and 1950 there were no fewer than 171 Castle Class locomotives built at the GWR Swindon Works with all but 16 of these being new builds. At the time the Castles were hailed as the most powerful locomotives on any British railway with the GWR displaying the Class prototype, "Caerphilly Castle" at the 1924 British Empire Exhibition. The locomotive's ability to sustain high speeds and remain economical was enviable.
The "Castles" soon earned a reputation for excellence and reliability, setting a record in 1932 for a 77 mile journey between Swindon and Paddington at an average speed of 81.68 mph. At the time this represented the World Record for steam traction. Withdrawal commenced in the very early 1950s and by December 1965 all of the Class had been 'retired'. Although all Castle Class locomotives were scheduled to be scrapped there were eight that survived including "Earl of Mount Edgcumbe" pictured above.